I've used OpenHallway with some successes. It's simple and gets the job done when used to capture specific use cases or tasks. The only issue I experienced was the limit of 20 minutes per recording was forgotten by some people who'd been focused on an app for 20 minutes and so they carried on using and narrating, thinking they were still helping us out and were non-plussed to find out their generous help was lost.
Others may have better answers if you specify whether you're looking to record short, scripted test cases or are trying to capture open feedback sessions. OpenHallway is solid for the former; for the latter, I've only been happy with desktop applications like Camtasia. But, if I'm going to bring a laptop preloaded with that to them and show them how to use it, I'd much prefer to watch over their shoulder and take notes instead.
Some others are at http://remoteusability.com/tools/